Process of carburizing brake drums



uly 37, 1934. A. L.. BOEGEHOLD Er AL '19957,077

PROCESS 0F GARBURIZING BRAKE DRUMS Filed July 15. 1932' gwn Wawy @vaPatented July 17, 1934 UNITED STATES PATENT OFFICE PROCEVISS OFCABBURIZING BRAKE DRUMS Delaware Application July 13, 1932, Serial No.622,266

lClaims.

This invention relates to brake drums, particularly to such drums as areused on motor vehicles.

An object of the invention is to produce a non- 5 scoring drum, onehaving a high degree of concentricity, and to make such a drum by asimple and inexpensive process.

Another object is to perform upon a drum cold formed from low carbonsteel such operations as to avoid the necessity for subsequent machiningto obtain concentricity and to provide for non-scoring characteristics.

Other objects and advantages will be understood from the followingdescription.

Fig. l of the accompamling drawing shows in transverse section a drumafter the first forming step of the operation.

Fig. 2 illustrates in vertical section a container havingdrums about tobe carburized. 2@ Fig. 3 shows in transverse section a drum after beingcarburized.

Fig. 4 illustrates in vertical section the final sizing step.

Fig. 5 illustrates in vertical section a portion or the nmshed drum.

Fig. 6 is a perspective of a drum ldisclosing a slight modication.

Modern automobiles require brake drums having non-scoringcharacteristics and having a 39 high degree of concentricity since theyare subjected to severe braking pressure.

The process of making drums from low carbon steel by cold forming iswell known. Such drums become highly strained in the forming process.Unless this strained condition is relieved by anhealing it prevents theattainment of the required high degree of concentricity of the brakingsurface without resort to a machining operation. Even when the brakingsurface of such a drum is made concentric by machining, it has atendency to go out of round in service due to residual strains.

If the cold formed drum is annealed to re move the strains set upduringthe forming proc- 15 ess and thereafter restruck in a die, .thedrum can be sized to a greater degree of concentrioity.

Such cold formed steel drums are, however, not sufficiently resistant toscoring to meet the severe bramng conditions in modern automobiles.Therefore, it is desirable to raise the carbon content of the brakingsurface of the drum. This may be done by the known method of heating the-drum with solid carbonaceous material or .by gas carburizing. If thisoperation be substituted for the above-mentioned annealing opera- (Cl.zii-152.2)

tion, two results are accomplished. `The cold forming strains areremoved and the braking surface is transformed into one having highcarbon steel. After this dual operation the drum may be sized bystriking in a die to secure the required high degree of concentricity sothat it is not out of round more than .005'l and will remain round inservice.

In carrying out the above operation it is found that the initial formingstep should bring the 55 drum diameter to .005" less than that of thefinished drum. The punch for the nal sizing should be of a diameterequal to the desired inside diameter of the finished drum, or .005"larger than the inside diameter of the initially 79 formed drum. Thisproduces a slight stretching of the drum during the final sizing step.The sizing operation should also include the formation of a bead in theback of the drum to remove any distortion in this part of the drumresulting from the second step in the process.

Owing to the elimination of a machining step to produce concentricity,it is possible to reserve for use as a braking surface the outermostlayer of the carburized surface which has been found so to havepreferred frictional and score-resisting characteristics as compared tothat layer slightly below the surface. 'Ihe surface of steel which hasbeen carburized contains a considerably higher concentration of theintensely hard iron carbide than is present at a depth of .010 below thesurface.

It has been found that for certain lining materials the drum structureat predetermined depths below the surface may be preferred to the actualsurface obtained by carburizing. For such linings it may be desired tomachine the drum surface to reach -the desired depth. If the drum is notto a high degree concentric, that machining operation will remove moremetal in some places than in others with the result that the brakingsurface lacks uniformity. For this reason the above described formingoperation is very much to be desired since at any depth the machiningoperation will produce a uniform surface.

This novel process, then, is a simple inexpensive way to make a light,non-scoring concentric drum without resort to the use of cast iron;without resort to costly non-scoring alloys; and .without resort to theprocess of adding non-scoring liners to previously formed drums. The newprocess comprises essentially three steps-(a) forming the drum to aslight under size; (b) a. no simultaneous carburizing and annealingstep;

and (c) a final sizing step. Between the second and third steps the dnnnis preferably cooled slowly rather than quenched. In this way casehardening is prevented.

The drawing shows the several steps and the operation. In Fig. 1 is adrum 9 of conventional shape having a flange 11, and preferably formedwith a hole 13 in its back wall.. This drum has been cold formed fromlow carbon plain or alloy steel having good cold forming properties. Theinner diameter of the drum is very slightly less than that of thefinished drum. Several of such drums are then subjected to the secondstep. To carburize the shoe-engaging surface only of the drum theremaining surface may be protected from the carburizing process. To thatend a plate 15 may be placed in the bottom of each drum leaving a space17 between the plate and the bottom of the drum. Each drum is thenfilled with carburizing compound 19 and covered with a plate 21. Aseries of such drums is then assembled on supports 25 within a heatresistant container 23 which is then subjected to heat. The heatconverts to a high 'carbon content the annular surface in contact withthe carbonaceous material.

'I'he gases may escape to the space beneath the plates 15 and outthrough the holes 13 and around the outer surface of the drum. As aresult of thistreatment the annular surface which is to be subject tothe frictional contact of the brake shoes is converted to high carbonsteel and the rest of the drum retains its low carbon characteristics.

If desired the conversion of the annular braking surface of the drum tohigh carbon steel may be done by the gas carburizing process. To effectlocal carburization by this process, the drum after the flrst formingoperation would be copper plated all over and then ground or machined onthe braking surface to remove the copper plate. The drum would then beheated in a gaseous carburizing atmosphere which would increase the icarbon content of the drum at the braking surface only.

The drums are then allowed to cool at a comparatively slow rate whichwill develop in the high carbon layer a surface composed essentially oflamellar pearlite with or without some free cementite. The heatingprocess simultaneously removes the strained condition produced in thedrum during the original forming process. In consequence, the frictionsurface is non-scoring and the removal of the strains avoids the dangerof the drum going out of round in service.

For the final sizing step a die 29 is used with a punch 31 to slightlystretcithe drumrfrom its under size, as shown by dotted lines,to itsfinal size, as shown by full lines in Fig. 4. Also, the die and punchare shaped to form a bead 33 which is intended to take up the slack orto provide space for the reception of the material caused by thedistortion produced in the carburizing step and shown at 27 in Fig. 3.

The configuration of this bead is immaterial. It may be radial, as shownin Fig. 5, or it may be annular, as vshown at 35 in Fig. 6.

The brake drum made by the above-described simple process possesses thedesired characteristics of hot formed high carbon or special Aalloysteel drums without the known attendantdisadvantages of such drums.Among these disadvantages may be mentioneda. The cost of material.

b. The excessive die upkeep cost.

e. Difiiculty of obtaining accurate size.

d. The necessity for additional cleaning operations.

e. Surface decarburization which must be removed by machining to obtainthe desired resist ance to scoring.

Drums made by the herein described new process possess score-resistingcharacteristics better than may be obtained in cold formed drums madefrom steel of intermediate carbon content.

Drums made from this new process are relieved from the stresses of thecold forming operations, so that the usual difiiculties of obtaining andmaintaining a true shape in cold formed drums are minimized.

While drums made by this process may be used as they come from the thirdstep of the operations, if it should be necessary to render such drumsadaptable to special brake linings, they may obviously be groundorvmachined or rolled for such purpose.

Furthermore, this novel process involves saving of time in manufacturebecause carburization is effected on a limited surface only, which per-1 mits the absence of poor heat-conducting 'material between thecontainer and the drum enabling the drum to reach the carburizingtemperature very quickly.

We claim:

1. 'Ihe method of making brake drums which consists in cold forming adrum from sheet metal to a dameter slightly less than that of thefinished drum, simultaneously annealing the formed drum and carburizingits friction surface only, and striking the carburized drum in a die tofinished dimensions.

2. The method of making a brake drum which consists in cold forming adrum of low carbon sheet steel to a diameter slightly less than that ofthe finished drum, annealing the drum and carburizing its frictionsurface only, cooling the carburized drum to produce a pearlite surfacein the carburized region, and sizing the drum by stretching and shapingit to its desired shape and size.

3. The method of making a brake drum which 'consists in cold forming thedrum from sheet material to a size and shape only slightly differentfrom the final shape and size, annealing the drum to remove the strainsproduced by the forming step and simultaneously carburizing the frictionsurface only, and thereafter striking the drum in a die to the requiredfinal dimensions.

4. The process of making carburized brake drums which consists inshaping a blank of sheet material into the form of a drum, forming ahole f shoe-engaging part only, sizing the drum, the

sizing step including the forming of a bead in the wall of the drum totake up the distortion produced in thecarburizing step.

ALFRED L. BoEGEHoLD. CLARENCE J. TOBIN. C. F. SMART.

